The field of the invention is silane chemistry and modification of metal surfaces.
Metal oxides and metals with oxide coatings are used to make various materials and components, including separation substrates for liquid and gas chromatography, substrates for capillary zone electrophoresis, biosensors, microelectronic devices, catalysts, fillers, and pigments. For many of these applications, it is desirable to modify the metal oxide surface, for example, by altering the adsorption, adhesion, wettability, or catalytic properties of the surface.
One way to modify a metal oxide surface is to attach to hydroxyl groups on the surface silane compounds having desired functional groups. Chlorosilanes and alkoxysilanes have been used for such surface modification. The use of these silane compounds can be problematic, however, because chlorosilanes and alkoxysilanes are moisture sensitive and sometimes act as corrosive agents. In addition, the reaction of chlorosilanes with metal oxide surfaces generates hydrochloric acid as a by-product, and the hydrochloric acid may corrode the modified metal oxide surfaces. Furthermore, some chlorosilanes and alkoxysilanes do not react with metal oxide surfaces.
In one aspect, the invention features a method of modifying a surface. The method includes contacting the surface with a hydridosilane under conditions and for a time sufficient to form a covalent bond between the silicon atom of the hydridosilane and the oxygen atom of a hydroxyl group on the surface. The hydridosilane has the formula: 
where each of Ra, Rb, Rc, and Rd is, independently, H, linear C1-30 alkyl, branched C1-30 alkyl, cyclic C3-30 alkyl, linear C2-30 alkenyl, branched C2-30 alkenyl, linear C2-30 alkynyl, branched C2-30 alkynyl, C6-20 aralkyl, C6-10 aryl, or a polymeric moiety having a molecular weight of about 1000 to about 100,000. The polymeric moiety is selected from the group consisting of hydrocarbon polymers, polyesters, polyamides, polyethers, polyacrylates, polyurethanes, epoxies, and polymethacrylates. Each of Ra, Rb, Rc, and Rd is optionally substituted with one or more substituents selected from the group consisting of xe2x80x94F, xe2x80x94Cl, xe2x80x94Br, xe2x80x94CN, xe2x80x94NO2, xe2x95x90O, xe2x80x94Nxe2x95x90Cxe2x95x90O, xe2x80x94Nxe2x95x90Cxe2x95x90S, 
xe2x80x94N3, xe2x80x94NReRf, xe2x80x94SRg, xe2x80x94ORh, xe2x80x94CO2Ri, xe2x80x94PRjRkRl, xe2x80x94P(ORm)(ORn)(ORp), xe2x80x94P (xe2x95x90O)(ORq)(ORs), xe2x80x94P (xe2x95x90O)2ORt, xe2x80x94OP(xe2x95x90O)2ORu, xe2x80x94S(xe2x95x90O)2Rv, xe2x80x94S(xe2x95x90O)Rw, xe2x80x94S(xe2x95x90O)2ORx, xe2x80x94C(xe2x95x90O)NRyRz, and xe2x80x94OSiRaaRbbRcc. Each of Re, Rf, Rg, Rh, Ri, Rj, Rk, Rl, Rm, Rn, Rp, Rq, Rs, Rt, Ru, Rv, Rw, Rx, Ry, and Rz, is, independently, H, linear C1-10 alkyl, branched C1-10 alkyl, cyclic C3-8 alkyl, linear C2-10 alkenyl, branched C2-10 alkenyl, linear C2-10 alkynyl, branched C2-10 alkynyl, C6-12 aralkyl, or C6-10 aryl, and is optionally substituted with one or more substituents selected from the group consisting of xe2x80x94F, xe2x80x94Cl, and xe2x80x94Br. Each of Raa, Rbb, and Rcc is, independently, linear C1-10 alkyl, branched C1-10 alkyl, cyclic C3-8 alkyl, linear C2-10 alkenyl, branched C2-10 alkenyl, linear C2-10 alkynyl, branched C2-10 alkynyl, C6-12 aralkyl, C6-10 aryl, xe2x80x94F, xe2x80x94Cl, xe2x80x94Br, or ORdd, where Rdd is linear C1-10 alkyl or branched C1-10 alkyl. At least one of Ra, Rb, Rc, and Rd is H and at least one of Ra, Rb, Rc, and Rd is not H. Preferably, two or three of Ra, Rb, Rc, and Rd are H.
The surface is preferably a metal surface. The metal surface can be selected from the group consisting of a titanium surface, a tin surface, an aluminum surface, an iron surface, a nickel surface, a chromium surface, a manganese surface,-a zirconium surface, a niobium surface, a molybdenum surface, or a tungsten surface. The surface can also be a metal oxide surface or a metallate surface. Alternatively, the surface can contain a metal alloy. A preferred embodiment includes forming a monolayer-modified metal surface.
In another preferred embodiment, each of Ra, Rb, Rc, and Rd is, independently, H, linear C1-30 alkyl, branched C1-30 alkyl, cyclic C3-30 alkyl, linear C2-30 alkenyl, branched C2-30 alkenyl, linear C2-30 alkynyl, branched C2-30 alkynyl, C6-20 aralkyl, or C6-10 aryl. Preferably, at least one of Ra, Rb, Rc, and Rd is linear C1-20 alkyl, or branched C1-20 alkyl, or phenyl. More preferably, at least one of Ra, Rb, Rc, and Rd is unsubstituted linear C1-20 alkyl, unsubstituted branched C1-20 alkyl, or unsubstituted phenyl.
The invention also features a method of forming a monolayer-modified metal surface that includes contacting a metal surface with a hydridosiloxane-containing polymer under conditions and for a time sufficient to form a covalent bond between at least one silicon atom of the polymer and an oxygen atom of a hydroxyl group on the metal surface. The polymer has the formula Rdd [xe2x80x94Oxe2x80x94Si (Ree)(Rff)]nxe2x80x94. Rgg, where each of Rdd and Rgg is, independently, C1-6 alkoxy or C1-6 alkyl, each of Ree and Rff is, independently H or C1-6 alkyl, and n is 10 to 1000.
In preferred embodiments, the polymer is a copolymer of hydridomethylsiloxane and dimethylsiloxane. Preferably, the copolymer is at least 50 mol % hydridomethylsiloxane, about 25 mol % to about 30 mol % hydridomethylsiloxane, or about 1 mol % to about 5 mol % hydridomethylsiloxane.
The invention also features a surface that includes a plurality of Mxe2x80x94Oxe2x80x94Sixe2x80x94(Ra)(Rb)(Rc) moieties. M is Ti, Sn, Al, Fe, or Ni. One or two of Ra, Rb, and Rc are H, and the remaining one or two of Ra, Rb, and Rc are, independently, H, linear C1-30 alkyl, branched C1-30 alkyl, cyclic C3-30 alkyl, linear C2-30 alkenyl, branched C2-30 alkenyl, linear C2-30 alkynyl, branched C2-30 alkynyl, C6-20 aralkyl, or C6-10 aryl, or a polymeric moiety having a molecular weight of about 1000 to about 100,000. The polymeric moiety is selected from the group consisting of hydrocarbon polymers, polyesters, polyamides, polyethers, polyacrylates, polyurethanes, epoxies, and polymethacrylates. Each of Ra, Rb, Rc, and Rd is optionally substituted with one or more substituents selected from the group consisting of xe2x80x94F, xe2x80x94Cl, xe2x80x94Br, xe2x80x94CN, xe2x80x94NO2, xe2x95x90O, xe2x80x94Nxe2x95x90Cxe2x95x90O, xe2x80x94Nxe2x95x90Cxe2x95x90S, 
xe2x80x94N3, xe2x80x94NReRf, xe2x80x94SRg, xe2x80x94ORh, xe2x80x94CO2Ri, xe2x80x94PRjRkRl, xe2x80x94P(ORm)(ORn)(ORp), xe2x80x94P(xe2x95x90O)(ORq)(ORs), xe2x80x94P(xe2x95x90O)2ORt, xe2x80x94OP(xe2x95x90O)2ORu, xe2x80x94S(xe2x95x90O)2Rv, xe2x80x94S(xe2x95x90O)Rw, xe2x80x94S(xe2x95x90O)2ORx, xe2x80x94C(xe2x95x90O)NRyRz, and xe2x80x94OSiRaaRbbRcc. Each of Re, Rf, Rg, Rh, Ri, Rj, Rk, Rl, Rm, Rn, Rp, Rq, Rs, Rt, Ru, Rv, Rw, Rx, Ry, and Rz, is, independently, H, linear C1-10 alkyl, branched C1-10 alkyl, cyclic C3-8 alkyl, linear C2-10 alkenyl, branched C2-10 alkenyl, linear C2-10 alkynyl, branched C2-10 alkynyl, C6-12 aralkyl, or C6-10 aryl, and is optionally substituted with one or more substituents selected from the group consisting of xe2x80x94F, xe2x80x94Cl, and xe2x80x94Br. Each of Raa, Rbb, and Rcc is, independently, linear C1-10 alkyl, branched C1-10 alkyl, cyclic C3-8 alkyl, linear C2-10 alkenyl, branched C2-10 alkenyl, linear C2-10 alkynyl, branched C2-10 alkynyl, C6-12 aralkyl, C6-10 aryl, xe2x80x94F, xe2x80x94Cl, xe2x80x94Br, or ORdd, where Rdd is linear C1-10 alkyl or branched C1-10 alkyl. Preferably, two of Ra, Rb, and Rc are H. A preferred surface is a monolayer-modified metal surface.
As used herein, xe2x80x9cmonolayer-modified metal surfacexe2x80x9d means a surface including a plurality of silane moieties, substantially all of which are covalently bonded to oxygen atoms, which are covalently bonded to metal atoms.
The methods of the invention can be used to modify surfaces that react with the hydridosilane compounds, yielding modified surfaces containing covalently attached organosilane moieties. Functional groups on the organosilane moieties can be chosen to affect the properties of the modified surfaces advantageously.
Using hydridosilanes to modify surfaces according to the invention offers several advantages. Generally, hydridosilanes have lower boiling points than corresponding chlorosilanes and alkoxysilanes. Furthermore, hydridosilanes are not moisture sensitive and are not corrosive. In addition, they do not form corrosive by-products when they react with most surfaces, including metal oxide surfaces. Instead, the byproduct of the reaction is hydrogen gas (H2). Consequently, hydridosilanes can be deposited using vapor phase techniques.
An exemplary reaction is the following: 
In the reaction shown above, a Mxe2x80x94OH group on a metal surface reacts with the hydridosilane to produce a Mxe2x80x94Oxe2x80x94SiRaRbRc moiety, where M, Ra, Rb and Rc are as described above. The by-product of the reaction is hydrogen gas. The resulting surface has silane moieties covalently bonded to oxygen atoms, which are covalently bonded to metal atoms. Because xe2x80x94OH moieties are replaced with xe2x80x94Oxe2x80x94SiRaRbRc moieties, the properties of the surface are altered.
Surfaces
Various surfaces can be modified according to the invention. The surfaces of titania powders, titania single crystals, titanium foils, and titanium films can be modified. Other metal surfaces, including tin, aluminum, iron, nickel, chromium, manganese, zirconium, niobium, molybdenum, and tungsten surfaces, can also be modified. Surfaces composed of oxides of one of these metals, for example, ceramic surfaces, can be modified as well. In addition, metallate surfaces, such as titanate, niobate, molybdate, or tungstate surfaces, can be modified. Surfaces that include alloys of these metals, for example, stainless steel, can be modified as well.
Hydridosilanes and Hydridosiloxane-Containing Polymers
Various hydridosilanes and hydridosiloxane-containing polymers can be used in the invention. Dihydridosilanes are silanes of Formula I in which at least two of Ra, Rb, Rc, Rd are H. Trihydridosilanes are silanes of Formula I in which at least three of Ra, Rb, Rc, and Rd are H. Hydrido substituents are smaller than silane moieties with bulky substituents such as branched alkyl substituents. Dihydridosilanes and trihydridosilanes consequently provide higher degrees of surface coverage than that provided by monohydridosilanes. Examples of useful trihydridosilanes include C8H17SiH3, C6F13(CH2)2SiH3, C6H5SiH3, CH2xe2x95x90CHxe2x80x94CH2SiH3, Br(CH2)3SiH3, and C13H37SiH3.
For making hydrophobic surfaces, hydridosilanes with bulky alkyl or alkenyl groups are preferred. Examples of silanes useful for hydrophobic surfaces include (i-Pr)3xe2x80x94SiH, t-BuSiMe2H, C18H37SiMe2H, and C6F13 (CH2)2SiMe2H.
In some embodiments, compounds containing more than one silicon atom are used. For example, polymers functionalized with one silane moiety per polymer to one silane moiety per monomer unit are sometimes used. Hydrocarbon polymers, such as polystyrene and polyethylene, functionalized with silane moieties can be used. In addition, functionalized polyesters, polyamides, polyethers, polyacrylates, polyurethanes, epoxies, and polymethacrylates can be used to modify surfaces.
In other embodiments, hydridosiloxane-containing polymers, such as copolymers of hydridomethylsiloxane and dimethylsiloxane, are used. Copolymers of hydridomethylsiloxane and dimethylsiloxane consist of monomeric units having the formulas [Si(H)(CH3)xe2x80x94Oxe2x80x94] and [Si(CH3)3xe2x80x94Oxe2x80x94]. Copolymers in which 3-50 mol % of the polymer is hydridomethylsiloxane are preferred. One advantage of using polymers instead of small molecules is that thicker monolayers can be formed when polymers are used.
Pretreatment of Surfaces
In some embodiments, it is desirable to pretreat the surfaces to ensure they are hydrated (i.e., hydroxylated) and clean, before forming the silane layer. Pretreatment can lead to higher surface coverage, more uniform surface coverage, or both. The surface pretreatment of planar substrates can be carried out as follows. A single crystal, foil, or film of titania is washed with water, a detergent such as sodium DDS, and/or an organic solvent. Useful solvents include methylene chloride, alkanes, diethyl ether, tetrahydrofuran, acetonitrile, ethyl acetate, benzene, ethanol, methanol and toluene. The substrate is then treated with a strong oxidizing agent, such as sulfuric acid, hydrogen peroxide, chromium acid, or oxygen plasma. Alternatively, the substrate can be heated in the presence of oxygen.
For disperse substrates (i.e., fine powders or porous materials with features of 1 xcexcm or less), the surface pretreatment can be carried out by placing the substrate powders, porous particles, or membranes under vacuum at elevated temperatures. High temperatures, for example, 100-200xc2x0 C., 20 mTorr, can be used. Alternatively, the substrate can be heated in the presence of oxygen.
Modification Reaction Conditions
Following the pre-treatment process, the surface is modified by a surface reaction with a hydridosilane. The modification reaction can be performed under various conditions. The modification reaction can be run in the vapor phase, in the liquid phase, or in supercritical fluids. In addition, the reaction can be performed in dilute or concentrated solutions, and at high or low temperatures.
In some embodiments it is advantageous to modify the surfaces using vapor phase deposition techniques, because the modification reactions are easier to perform in the vapor phase. An additional advantage is that when vapor phase techniques are used, fewer side products are generated. Surface modification using vapor phase techniques can be achieved by placing the substrate in an environment saturated with the desired hydridosilane vapor. The reaction mixture is maintained at room temperature or at an elevated temperature, for example, 100xc2x0 C., for a duration ranging from several hours to several days. The temperature used is determined by the vapor pressure of the silane used.
Liquid phase modification is preferred for hydridosilanes with high boiling points, e.g., polymers, high molecular weight oligomers and octadecyldimethylsilane. Liquid phase modification can be carried out as follows. The substrate is covered with a solution of hydridosilane in an inert organic solvent. Solvents that may be used include liquid alkanes, benzene, and toluene. Alternatively, the substrate is covered with neat hydridosilane. The reaction mixture is maintained at room temperature or an elevated temperature for a period of time ranging from several hours to several days.
In general, the reaction between the hydridosilane and the hydroxyl groups on the surface occurs rapidly, at low temperatures. When the reaction occurs rapidly, heating, which can lead to poor monolayer packing, is not necessary. Hindered hydridosilanes (e.g.,tri-isopropylsilane and t-butyldimethylsilane) do not react with some surfaces at low temperatures, or react extremely slowly, so heating is necessary when these silanes are used. In many cases, 80% of the surface coverage is formed after one hour. The reaction time can be extended, e.g., to 24 hours, if denser coverage is desired.
Properties of Modified Surfaces
The invention can be used to improve adhesion to titania and oxidized titanium surfaces and for the lyophobization of titania and oxidized titanium surfaces. The methods are therefore useful for the surface modification of titania adsorbents, catalysts (e.g., photocatalysts), membranes, and chromatographic stationary phases. They are also useful for the preparation of chemically modified titania electrodes and sensors and for the preparation of well characterized self-assembling monolayers on single crystals of titania and on oxidized titanium surfaces. Specific uses include dental and medical implants and printing plate applications.
The modified surfaces have silane moieties covalently attached to the surfaces. The substituents of the silane moieties can be chosen to give the modified surfaces desired properties. For the hydrophobization of surfaces, silane moieties including unsubstituted linear or branched alkyl groups or aryl groups are preferred. For adhesion promotion, silane moieties in which R1-R4 are alkenyl groups or aryl groups are preferred. The alkenyl or aryl groups may be substituted with hydridosilyl groups, amino groups, chloro groups, bromo groups, alkoxy groups, or carboxy groups. For the oleophobization of surfaces, silane moieties in which R1-R4 are C1-3 alkyl groups, branched alkyl groups, fluoroalkyl groups, alkylsiloxanes, or fluoroalkylsiloxanes are preferred.
For the surface modification of titania based electrodes, silane moieties in which R1-R4 are substituted with amino, chloro, bromo, alkoxy, carboxy, and hydridosilyl groups are preferred. For the preparation of titania chromatographic stationary phases and membrane applications, silane moieties in which R1-R4 are C8-22 alkyl or C1-10 aryl are preferred. The alkyl or aryl groups may be substituted with amino or carboxy groups.
In order that the invention may be more fully understood, the following specific examples are provided. The examples do not limit the scope or content of the invention in any way.